Demystifying
By Brian Rohrig
ere is some good news for you:
You can blame the sounds and
odors that come from your body
on bacteria. Yes, these little critters—which live on our skin, in our mouths,
and in our guts—are the ones responsible for
a lot of what is going on inside our bodies.
Since today’s society is quite obsessed with
cleanliness, we tend to be a little uptight about
all these bodily sounds and smells. But understanding the science behind what may appear
to be so gross may make it, well… less gross.
FIGURES BY anthony fernandez. PHOTOS FROM ISTOCK
ISTOCK
What’s on your nose?
Probably every teenager
has looked in the mirror
first thing in the morning
to discover a ginormous
zit staring back at them.
And it usually makes its
appearance at the worst
possible time, such as the
day of the prom.
Your skin is porous—it is filled with millions
and millions of tiny little holes, or pores. Hair
grows out of pores known as follicles. In the
skin, glands release an oily substance that is
pale yellow and that lubricates and protects
the skin (Fig. 1).
This oily substance, called sebum, is one of
the main causes of acne. Although it is essential to keep our skin soft and pliable and our
hair shiny, too much of it can be a problem.
Especially in teenagers, large levels of a sex
hormone, called testosterone, are produced,
12 Chemmatters, OCTOBER 2011
Hair
Skin surface
Sebum
Follicle
Sebaceous
gland
Figure 1. Sebaceous glands in the skin release
an oily substance called sebum that lubricates
and protects the skin.
up below the surface of the skin, and die.
These dead white blood cells, along with
dead skin cells and some bacteria, form a
white liquid known as pus. A pimple forms
when the excess sebum and dead skin cells
clog up and block the opening of the pore. This
type of pimple is called a whitehead (Fig. 2a).
Another type of pimple, called a blackhead
(Fig. 2b), appears when sebum and dead skin
cells clog the pore but not the opening, as in a
whitehead. While the pore is clogged, its surface remains open. A blackhead appears black
because melanin in the dead skin cells reacts
with oxygen from the air, which changes the
melanin’s color from brown to black.
If the infection worsens, a painful cyst may
develop under the skin. A cyst is a fluid-filled
sac that is the most severe type of acne. It can
cause permanent scarring.
A majority of teenagers have acne, some
worse than others. Washing your face with
soap and water several times each day is a
good way to minimize acne.
which causes the skin to release a lot of
sebum, too. Sometimes, this excess sebum
can clog up the pores.
Adding to the mix are dead skin cells. About
30,000 skin cells are shed every minute! A lot
of these skin cells are shed inside the pores
themselves.
A blocked skin pore also contains bacteria.
They feed off the dead skin cells and the
clogged sebum within the pores and produce
toxins that damage the lining of the pores. As
these bacteria grow and multiply, they invade the area surSkin surface
Skin surface
rounding the pore, which can
Blackhead
Postule
lead to a bacterial infection.
Enlargement
Enlargement
of follicle
A blocked pore initially turns of follicle
opening
opening
red because blood rushes to
Sebaceous
Sebaceous
gland
the site, which is one of the
gland
ways our body responds to an
Follicle
Follicle
infection. Then, white blood
cells—a type of blood cell
(a)
(b)
responsible for fighting infecFigure 2. Two types of pimples: (a) a whitehead; and (b) a blackhead
tion—destroy bacteria, build
www.acs.org/chemmatters
Soap is an amphiphilic molecule: One end
of the molecule binds to water molecules,
while the other end binds to fat molecules,
such as the ones present in sebum. These
ends are called “water-loving” and “oil-loving,” respectively—hence the word “amphiphilic” (from the Greek “amphis,” meaning
“both” and “philia,” meaning “love”).
The reason some molecules bind to water
and others to oil—and sometimes to both—
is due to one key difference between water
and oil, called polarity. Water is polar; oil is
nonpolar.
A polar molecule contains regions of partial
positive and negative charge that are due to
the uneven distribution of electrical charge. In
the case of a water molecule, the oxygen end
has a greater concentration of electrons than
the two hydrogen ends because oxygen tends
to attract shared electrons while hydrogen
tends to lose them. So, even though the water
molecule is electrically neutral, it contains a
partial negative charge on the oxygen end and
a partial positive charge on each of the hydrogen ends (Fig. 3).
A nonpolar molecule has an even distribution of electric charge, so it has no regions
of partial positive and negative charges. This
is the case with an oil molecule, in which the
electrons are shared equally between the molecule’s atoms.
Polar molecules bind to other polar molecules or to ions. Nonpolar molecules bind
to other nonpolar molecules. Polar and nonpolar molecules cannot bind with each other
because one has a partial electric charge but
not the other.
A common component of soap is sodium
stearate [CH3–(CH2)16–COONa]. When this
molecule dissolves in water, it produces a
stearate anion (C17 H35COO–) and a sodium
cation (Na+). The stearate anion is amphiphilic: It has a negatively charged end and a long,
nonpolar tail (Fig. 4).
The negatively charged end binds to the
electrically positive regions of water molecules, and the long tail binds to sebum
molecules. Then, the soap molecules group
together to form tiny spheres called micelles
CH3
(CH2)16
C
O—
Nonpolar tail
Figure 4. When dissolved in water, sodium
stearate—the soap that most of us use—
decomposes into an anion (top) and a cation
(Na+) (not shown). The anion has a polar head
that binds to water and a nonpolar tail that binds
to the skin’s oily substance. It is this anion that
helps clean pimples by washing away the skin’s
oily substance.
sive problem: bad
breath. Your mouth is
actually teeming with
about 10 billion bacteria that take in food
and excrete waste.
Bacterial waste is in
the form of gas, which
can be pretty smelly.
Bad breath is caused
by the combined
waste products of
these bacteria.
... bacteria
While you sleep,
multiply like
your body stops producing saliva, which
crazy at
contains antibacterial
night, and
compounds. In the
your breath
absence of this cleanscan smell
ing liquid, bacteria
Smelly breath
pretty bad in
multiply like crazy at
the morning.
If you wake up and are obsessing over
night, and your breath
that zit on the end of your nose, you may
can smell pretty bad in
not even notice a potentially more offenthe morning.
During the day, your body produces up to a
liter and a half of saliva, which keeps bacteria
Nonpolar
δ–
Region
in check and washes away food particles
O
+
δ
+
H
H
H
δ
that may be lodged in your teeth. If you don’t
δ+ δ+
O
CO H O
OOC
δ–
–
δ
brush your teeth before going to bed, the bac+
O
δ
δ+
H
H
H
COO H
teria have a lot more food particles to munch
+
+
δ
δ OOC
O
–
δ
on at night, leading to a lot of bacterial waste.
δ–
O
One of the best ways you can prevent
OILY
COO δ+ δ+
H
H
SUBSTANCES
H
H
breath odor—and strengthen your teeth—is
δ+ δ+ OOC
O
by using mouthwash, which kills the bacteria
δ–
–
δ
in your mouth. One key ingredient in many
O
COO +
δ
H
δ+
H + δ+ OOC
H
mouthwashes is fluoride, which is known to
H
δ
–
O
δ
COO +
strengthen tooth enamel (Fig. 6).
–
δ
δ
O H
C
δ+
+ OO
H
H
Polar
Fluoride (F–) is the ionic form of fluorine. It
H+ δ
δ
O
Region
–
δ
forms when a fluorine atom gains an electron.
Fluoride does not exist by itself, but it can be
Figure 5. After binding to oily substances from the
skin, sodium stearate anions pack together to form
found in compounds, such as sodium fluoride
a sphere called a micelle (top). The negatively
(NaF), which is present in many toothpastes
charged ends are in contact with water while the
long tails trap the oily substances inside the micelle. and mouthwashes. When this compound is
(Fig. 5). A micelle consists on the
outside of the negatively charged ends
attached to water molecules and on the
inside of the nonpolar tails attached to
sebum molecules. So, the sebum molecules are essentially stuck inside the
micelles and can be washed away.
But too much washing can dry out the
skin, causing the skin to produce more
sebum. Also, once acne has developed,
no amount of washing will remove it.
At that point, acne is due to a bacterial
infection, and only antibacterial agents
can treat it. Antibacterial substances
destroy or prevent the growth of bacteria.
They are available in the form of overthe-counter creams in pharmacies or are
prescribed by a dermatologist.
ISTOCK
H
δ+
O
Mike ciesielski
H
δ+
Polar head
–
–
–
–
–
–
–
anthony fernandez
δO
Figure 3. In a water
molecule, the electrons are
shared unevenly between
the oxygen end and the two
hydrogen ends, creating a
partial negative charge on
the oxygen end and a partial
negative charge on the
hydrogen ends.
–
–
–
chemmatters, OCTOBER 2011 13
Marie dauhenheimer
Ca5(PO4)3(OH) à 5 Ca2+ + 3 PO43– + OH–
A certain amount of demineralization is
normal. But it is also normal for the reverse
process, remineralization, to occur:
5 Ca2+ + 3 PO43– + OH– à Ca5(PO4)3(OH)
If too much bacterial acid is produced,
demineralization can outstrip mineralization,
leading to a cavity. How does this happen?
When acids are present in a solution, they
dissolve to produce hydrogen ions (H+). In the
mouth, as bacteria produce acids, the amount
of hydrogen ions builds up. These ions combine with the hydroxide ions produced during
demineralization to form water:
PHOTOS.COM
H+ + OH– à H2O
But hydroxide ions are essential to remineralization, so their neutralization by hydrogen
ions causes remineralization to slow down. The
hydroxyapatite on the surface of the teeth keeps
dissolving, ultimately leading to tooth decay.
Fluoride ions present in mouthwashes help
the enamel to remineralize. They accumulate
on the surface of the enamel, thus creating
a barrier that prevents bacterial acids from
reaching the enamel. Also, the fluoride ions
attract calcium ions, ultimately changing hydroxyapatite into fluoroapatite
[Ca5(PO4)3F], which is stronger than the
original hydroxyapatite.
Bad breath can be caused by many different
14 Chemmatters, OCTOBER 2011
CH2OH
CH2OH
gases, but two of
the most common
C
O
C
H
H
ones are hydrogen
H
H
sulfide (H2S) and
O C
C
C
H
OH
OH
methyl mercaptan
C
C
C
H
(CH3SH)—both
sulfur-containing
H
H
OH
compounds. Other
Figure 7. Chemical structure of cellulose
gases that lead to
bad breath are indole
(C8H7N) and skatole (C9H9N), the two gases
primarily responsible for the smell of feces.
CH2OH
O
C
www.acs.org/chemmatters
C
O
C
H
C
H
OH
H
C
C
H
OH
H
OH
Passing gas
Eating a lot of fiber can have an undesirable
side effect: the production of large amounts
of intestinal gas. When this gas is released
by the body, it is known as flatulence. The
gas itself is known as flatus. “Passing gas” is
actually a good way to describe this process.
People pass gas 14 times per day, on average.
This gas is produced by bacteria in the colon.
Fiber is made of
a substance called
cellulose (Fig. 7).
Cellulose belongs to
a group of materials
called carbohydrates
that are composed of
carbon, hydrogen,
and oxygen and are
made of a series
of repeating small
molecules. In the
People pass
case of cellulose,
gas
14 times
the repeating
per day, on
small molecule is
glucose (C6H12O6)
average.
(Fig. 8).
In the colon,
bacteria break down cellulose, so if undigested food enters the colon, there is more for
the bacteria to feed on. And when you have
a lot of bacteria, you have a lot of their waste
products in the form of gas. Foods high in
fiber—such as fruits, vegetables,
and beans—tend to produce a
lot of flatulence.
Some indigestible sugars
can have the same effect.
For instance, lactose
in milk, which is
a carbohydrate
molecule (Fig.
9) formed from
glucose and galac-
H
O
C
O
H
CH2OH
H
OH
O
H
OH
H
H
OH
OH
H
Figure 8. Chemical structure of glucose
CH2OH
O
HO
H
OH
H
H
OH
CH2OH
H
O
O OH
H
OH
H
H
OH
Figure 9. Chemical structure of lactose
SHUTTERSTOCK
dissolved in water,
the fluoride ions
are free to move.
Fluoride ions
Dentin
prevent tooth
decay by strengthening the enamel.
The primary comNerve
pound found in
tooth enamel is a
strong, insoluble
Figure 6. Enamel is the
mineral called
hard white sustance that
covers a tooth.
hydroxyapatite
[Ca5(PO4)3(OH)].
Hydroxyapatite contains positive ions (Ca2+)
and negative ions (PO43– and OH–), which are
attracted to each other to form the crystalline
structure of hydroxyapatite.
The bacteria present on our teeth produce
acids that cause hydroxyapatite to break
apart—a process called demineralization:
Enamel
tose molecules, is sometimes not broken
down completely. So, dairy products can
produce a lot of flatulence, especially if a
person is lactose intolerant.
Once the basic chemical reactions of
the body are understood, the “gross”
things of your body won’t seem all that
gross. On a molecular level, no one compound is grosser than any other. It’s all
just chemistry!
Check out the video podcasts on acne and
bad breath at: www.acs.org/chemmatters
Selected references
Masoff, J. Oh, Yuck! The Encyclopedia of
Everything Nasty, Workman Publishing: New
York, 2000.
Bad Breath: Unusual Causes of Halitosis. Health
Tree: http://www.healthtree.com/articles/halitosis/bad-breath-causes/ [accessed July 2011].
What Causes Acne? Skin Care Physicians, April
14, 2010: http://www.skincarephysicians.com/
acnenet/acne.html [accessed July 2011].
Brian Rohrig teaches chemistry at Jonathan Alder
High School in Plain City (near Columbus), Ohio.
His most recent ChemMatters article, “Myths:
Chemistry Tells the Truth,” appeared in the
December 2010 issue.